Keep unreferenced buffers around

One problem with delaying destruction of buffer objects is that the
compositor may create a shadow that references defunct buffers.

One way to fix that issue is to immediately destroy buffers. However,
there is other way to address the issue - keep released buffers alive.

If a buffer is kept alive by the client, then it will most likely be
used again. It also simplifies buffer management.

BUG: 425233

src/wayland/server/buffer_interface.cpp

@@ -240,7 +240,6 @@ void BufferInterface::unref()
             wl_buffer_send_release(d->buffer);
             wl_client_flush(wl_resource_get_client(d->buffer));
         }
-        deleteLater();
     }
 }